We describe a novel approach for probing the development, architecture, and function of the mammalian cerebral cortex and related areas. Four gene trap lines of the mouse have been isolated in which the beta-galactosidase reporter gene is expressed in limited areas of the adult brain. In one line, the reporter is expressed uniformly throughout the neocortex. In a second, the expression is found primarily in anterior neocortical cells while a third line shows expression primarily in posterior neocortical cells. Neocortical staining is absent in the fourth line, but the piriform cortex shows pronounced staining. We propose more detailed histochemical characterization of the lines to elucidate developmental expression profiles and to better understand the adult cell types, layers, and regions that exhibit expression. We will isolate partial cDNAs representing the trapped genes and sequence these to obtain insights into cellular functions. RNA in situ hybridization, immunohistochemical characterization, and RNA blotting/protection experiments will be performed to verify that the reporter gene is expressed in a manner similar or identical to the intact gene and to determine whether the expression of the trapped genes is compromised in the various lines. Although no phenotypes have yet been observed, we will employ a battery of behavioral tests to assess cortical function. Finally, given that our initial gene trap screen has been quite successful in producing lines with interesting staining patterns, we propose a more extensive screen for gene expression patterns in the cortex utilizing a new type of gene trap vector in which the reporter gene product will be localized to the axons of the expressing cells. Together, these experiments comprise a novel approach to the development, structure, and function of the cortex beginning with a focus on gene expression patterns.